DESCRIPTION: Cells from individuals with the disease Xeroderma pigmentosum (XP) are defective in different facets of nucleotide excision repair (NER) with the exception of XP-variant (XP-V) individuals, which appear to have normal NER. The most commonly accepted hypothesis for XP-V is that it is due to a defect in a polymerase-related gene that leads to error-prone bypass of lesions. The principal investigator wishes to clone the XP-V gene in order to better understand the molecular basis of this defect. Evidence is presented bearing on the mechanism(s) responsible for the genetic predisposition to sunlight-induced skin cancer found in xeroderma pigmentosum variant (XP-V) patients, who develop clinical features of classical XP patients yet have normal excision repair. Their cells are abnormally slow in replicating UV-damaged DNA and are extremely sensitive to UV-induced mutations. Data has been obtained that suggest that this extreme hypermutability reflects a defective, error-prone replication complex that yields an abnormal spectrum of UV-induced mutations. Not only is replication in the intact cells abnormally error-prone but this is also true for in vitro replication of an irradiated template by cell-free extracts from XP-V cells. They represent the only known example of human cells with a defective error-prone replication complex not resulting from defective DNA repair. Thus, they offer an opportunity to investigate the mechanisms normal cells use to insure high-fidelity DNA replication. It is proposed to use two approaches simultaneously to clone the gene(s) responsible for the abnormal characteristics of XP-V cells and then to characterize the gene identified and its protein product.